Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 62. Improper Use of Flags or Military Emblems, Insignia or Uniforms of the Adversary
The UK Military Manual (1958) describes as treachery calling out “Do not fire, we are friends”, and then firing, noting that this “device is often accompanied by the use of enemy uniforms”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 311, footnote 1.
The manual also states: “If, owing to shortage of clothing, it becomes necessary to utilise apparel captured from the enemy, the badges should be removed before the articles are worn.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 322.
The manual further states:
The employment of the national flag, military insignia or uniform of the enemy for the purpose of ruse is not forbidden, but the [1907 Hague Regulations] prohibit their improper use, leaving unsettled what use is proper and what use is not. However, their employment is forbidden during a combat, that is, the opening of fire whilst in the guise of the enemy. But there is no unanimity as to whether the uniform of the enemy may be worn and his flag displayed for the purpose of approach or withdrawal. Use of enemy uniform for the purpose of and in connection with sabotage is in the same category as spying. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 320.
Furthermore, the manual states: “In addition to ‘grave breaches’ of the 1949 [Geneva] Conventions, … the following are examples of punishable violations of the laws of war, or war crimes: … use … of enemy uniform by troops engaged in a battle.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(f).
Lastly, the manual states: “Although no such opportunities of closing with the enemy by exhibiting his flag are possible in land warfare as in the case of naval warfare, national flags might be used to mislead the enemy.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 321.
The UK LOAC Pamphlet (1981) provides: “It is forbidden … to make improper use in combat … of the enemy’s national flag or uniform.” 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 12, § 2d.
The UK LOAC Manual (2004) states:
It is prohibited… to make use of the flags or military emblems, insignia or uniforms of adverse Parties while engaging in attacks or in order to shield, favour, protect or impede military operations … The prohibition on the use of such items of the enemy uniforms only arises in connection with actual military operations, so there would be no objection to enemy uniforms being worn in rear areas for training purposes or by a prisoner of war to facilitate his escape. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.11.
In its chapter on maritime warfare, the manual further states: “Warships and auxiliary vessels … are prohibited from launching an attack whilst flying a false flag, and at all times from actively simulating the status of those vessels exempt from attack.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 13.82.
Lastly, in its chapter on enforcement of the law of armed conflict, the manual states:
The Hague Regulations 1907 are now recognized as part of customary law. Those regulations provide that the following acts are “especially forbidden”:
f. to make improper use of a flag of truce, of the national flag or of the military insignia and uniform of the enemy, as well as the distinctive emblems of the Geneva Convention. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.27.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(vii) of the 1998 ICC Statute. 
United Kingdom, ICC Act, 2001, Sections 50(1) and 51(1) (England and Wales) and Section 58(1) (Northern Ireland).